Ionic conductivities in mixtures of N,N-dimethylformamide with water

Abstract

Mixtures at 25 °C of N, N-dimethylformamide (DMF) and water have been the solvents for 13 conductance runs involving the solutes CsCl, KCI and KNCS. The table below summarizes the solutions and solvent compositions for which limiting equivalent conductances have been determined. A flaskcell has been designed to facilitate mixing during a run. The limiting equivalent conductances have been evaluated with the full Fuoss-Hsia conductivity equation. The Robinson-Stokes equation, used for the estimation of limiting equivalent conductance, is shown to give very similar values to those given by the Fuoss-Hsia equation. Also presented are the corresponding values of the parameters a (the ion-size parameter) and KA (the association constant). The values obtained for KA are small. Plots of log KA (KCI) against reciprocal of dielectric constant and against the logarithm of water concentration show maxima. In the latter case, some evidence is presented that this indicates the participation of both water and DMF in the formation of ion-pairs in DMF/water mixtures The autogenic rising boundary method has been used to determine, in DMF/water solvents at 25 °C, the limiting cationic transport numbers of KNCS in mixtures containing 0.5 and 0.75 mole fraction of DMF. Within experimental error, no concentration dependence of transport number has been detected in the former solutions; in 0.75 mole fraction of DMF, slight concentration dependence has been observed. In the latte¡ solvent, corrections for ionic association are within the experimental error. Limiting ionic equivalent conductances derived from the above transport numbers and conductances are presented. The corresponding Stokes radii have been calculated. Plots against l0OlD of the Stokes radii of K+, Cl-, Cs* and NCS- are presented for complete range of DMF/water solvents at25 °C.The data obtained in this research for K+ and Cl- confirm the suspected shape, in DMF-rich regions of the existing plots for these ions. The shape of the Stokes radius plot for Cs* resembles that of f+. R[r* values calculated from the slope and from the intercept are anomalous. The plot for NCS-, although lirnited to four points, exhibits a similar shape to the plot for Cl-. Tentative R- values have been calculated. A search for correlations between solute and solvent properties in DMF/water mixtures has been conducted. Densities, ionic equivalent conductances, viscosities and excess volumes of mixing have been plotted against solvent composition. Free volumes of the solvent mixtures have been calculated and used as the abscissae for plots of ionic conductances, viscosities and Walden Products. No simple correlations have been observed. However, the plot of viscosity, against solvent composition provides evidence for the existence of at least one DMF-water complex. The 'hard sphere' volume of this complex has been estimated by two methods and a molecular formula has been proposed. A published theory which attempts to explain variations in Walden Product with solvent composition has been described and tested. Data for Cl- in DMF/water mixtures at 25 C indicate that the theory does not hold in this system.